RELEASE OF CALRETICULIN FROM NEUTROPHILS MAY ALTER C1Q-MEDIATED IMMUNE FUNCTIONS

Calreticulin is an abundant intracellular protein which is involved in a number of cellular functions. During cytomegalovirus infection, as well as inflammatory episodes in autoimmune disease, calreticulin can be released from cells and detected in the circulation, where it may a ct as an immunodominant autoantigen in diseases such as systemic lupus erythematosus. Calreticulin is known to bind to the molecules of inna te immunity, such as Clq, the first subcomponent of complement. Howeve r, the functional implications of Clq-calreticulin interactions are un known. In the present study we sought to investigate, in greater detai l, the interaction between these two proteins following the release of calreticulin from neutrophils upon stimulation. In order to pinpoint the regions of interaction, recombinant calreticulin and its discrete domains (N-, P- and C-domains) were produced in Escherichia coli. Both the N- and P-domains of calreticulin were shown to bind to the globul ar head regions of Clq. Calreticulin also appeared to alter Clq-mediat ed immune functions. Binding of calreticulin to Clq inhibited haemolys is of IgM-sensitized erythrocytes. Both the N- and P-domains of calret iculin were found to contain sites involved in the inhibition of Clq-i nduced haemolysis. Full-length calreticulin, and its N- and P-domains, were also able to reduce the Clq-dependent binding of immune complexe s to neutrophils. We conclude that calreticulin, once released from ne utrophils during inflammation, may not only induce an antigenic reacti on, but, under defined conditions, may also interfere with Clq-mediate d inflammatory processes.